1. Field of the Invention
This invention relates to antistatic layers applied to hydrophobic organic polymer films, especially photographic polyester film intended for use as a support for photographic film.
2. Background Art
Photographic films, in which a silver halide emulsion is coated on a polyester film support, have a high dielectric constant, and tend to develop static charges when they move in contact with surfaces of similar high dielectric constant. As a result, they can become fogged by the light generated by static discharge, even though this is not visible to the naked eye or easily detected by instrumentation.
U.S. Pat. No. 2,327,828 "Colloidal Carbon Antihalation Layer" (1943) discloses applying a plastic coating containing a carbon pigment to a film support for a photographic emulsion in order to decrease the buildup of static charges. U.S. Pat. No. 3,072,483 "Photographic Element Comprising Polyethylene Terephthalate Film Base" (1963) and U.S. Pat. No. 3,753,765 "Conductive Carbon Antistatic Backing for Photographic Film" (1973) disclose applying to an energy-treated polyester film support, preferably polyethylene terephthalate, a layer of an antistatic composition coated from an aqueous dispersion of an acrylic binder; in the case of U.S. Pat. No. 3,753,765 the binder may also contain carbon black. Inasmuch as polyester films, such as polyethylene terepthalate films, present a hydrophobic surface, the usual practice is to apply an intermediate "subbing layer" to the polyester surface in order to effect good adhesion between it and the antistatic coating. The subbing layer is, typically, a chlorine-containing copolymer resin coating which is applied to the polyester film support after the latter has been cast into film but before it has been stretched in two directions to obtain the desired biaxial orientation, and then heat set.
The trouble with this practice is that a certain amount of scrap film is formed in the process of biaxial orientation and heat setting of the polyester film, and it cannot be recycled because the subbing layer has already been applied, and is incompatible with the composition of the molten polyester. This prevents recycle of the scrap polyester film unless the subbing layer is removed beforehand, and its removal is costly and difficult, making this undesirable. As a result much film has to be discarded instead of being recycled. The present invention provides a solution to this problem.